Flywheel systems have been used for many years for storing energy in a system, and then releasing that stored energy back into the other system. Flywheel systems provide a smoothing effect to the operation of internal combustion engines and other kinds of power equipment. More recently, flywheel systems are being used in electrical applications for uninterruptible power supplies, UPS, by storing and releasing energy. Electrical energy storage flywheel systems, which include a flywheel rotor and an attached motor/generator, convert electrical energy to mechanical energy by using the motor to accelerate the flywheel rotor. The energy is stored kinetically in the motion of the flywheel rotor. Mechanical energy is then later converted back to electrical energy when required by using the generator to decelerate the spinning flywheel rotor. Using flywheel systems instead of conventional electrochemical batteries for electrical energy storage offers the advantages of potential higher reliability, longer life and much higher power capability if desired.
Earlier flywheel UPS systems used flywheel rotors constructed of steel due to simplicity, however the performance of such flywheel rotors are low with tip speeds typically limited to around 400 m/sec or less. In comparison, a flywheel rotor comprising composite flywheel rims can be operated at much higher speeds (700–1000 m/sec). The higher performance is the result of the increased strength to weight ratio and also to some extent the capability for more efficient mechanical property tailoring that is possible when using high strength glass and carbon fibers. Because the energy stored for a given flywheel design is proportional to the square of the tip speed but is only linearly proportional to the flywheel rotors(rim and hub)mass, researchers have fervently pursued the much the higher speed composite flywheel rims.
To be competitive in the power quality and reliability industry, flywheel based UPS systems must compete against conventional electrochemical battery based UPS which have low reliability but low cost. To compete effectively, high performance composite flywheel rims must be manufactured at low cost. Among the many of composite fabrication methods, it is generally accepted that filament winding offers the greatest potential for composite flywheel rims. Filament winding can be a highly automated process that is capable of high-speed material deposition and very high part quality, which is inherently needed for highly stressed flywheel rim. Filament wound flywheel rims are typically designed as thick, predominately hoop wound composite rings that can be spun to very high speeds and are hence very effective for energy storage.
One of the key issues to have workable composite flywheel rim is strain matching between the ID of the rim and OD of the hub. Various ways have been used to achieve this strain matching. For example, it is well known that various fibers can be used for making a composite flywheel rim to match the ID growth of a rim to the OD growth of a hub. Generally, fiber having lower elastic modulus is placed inside and fibers having higher elastic modulus on outside. In the case of glass/carbon fiber hybrid composite rim, the mixture ratio between glass and carbon fibers can be determined by considering not only relationship between hub OD growth and rim ID growth but also rotor cost. More glass fibers results in lower cost, but a greater mismatch of the diameter growths could be generated during rotor spinning.
The first example of glass/carbon hybrid rim is a rim in which all glass fiber composite is on the inside and all carbon fiber composite is on the outside. This rotor can be easily made by in situ curing filament winding technology proposed in utility patent application Ser. No. 09/951844. The drawback of a rim made of all-glass fiber layers and all-carbon fiber layers is stress and strain discontinuities at the interface between glass and carbon composites which may result in possible cracks during fabrication and operation.
On the other hand, commingling the glass and carbon fibers has been proposed to avoid the discontinuities at the interface above mentioned in the composite industry. In other words, the second example is a composite rim in which the mixture ratio of carbon fiber versus glass fiber is increased continuously in the radial direction, and the carbon and glass fiber filaments are uniformly dispersed microscopically. This can be a desirable solution, however it is not easy to make a rim like this and must result in high cost.
The other practical solution is a rim in which the mixture ratio of carbon fiber versus glass fiber increases incrementally from inside toward outside of the rim. That is, the ratio of the carbon fiber versus glass fiber is constant in each layer and can be determined as a function of the number of tows within a fiber band during filament winding. When winding layers with carbon fiber tows and glass fiber tows, it is convenient to lay down a large number of tows together in a band with each revolution of the mandrel on which they are being wound. However, it has been discovered that occasionally, by chance, the carbon fiber tows in the band are laid down in a radially aligned pattern, as shown in FIG. 1, which produces an undesirable distribution of glass and carbon fibers and could result in large internal shear forces between the aligned or stacked carbon fiber regions and the adjacent aligned or stacked glass fiber regions.